Fuel cell power systems convert a fuel and an oxidant to electricity. One type of fuel cell power system employs a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of the fuel (such as hydrogen) and the oxidant (such as air or oxygen) to generate electricity. Water is a byproduct of the electrochemical reaction. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of a stack of fuel cells normally deployed in a fuel cell power system.
In the typical fuel cell stack, the individual fuel cells have fuel cell plates with channels through which various reactants and cooling fluids flow. Fuel cell plates may be unipolar, for example. A bipolar plate may be created by combining a pair of unipolar plates. Movement of water from the channels to an outlet header through a tunnel region formed by the fuel cell plates is caused by the flow of the reactants through the fuel cell assembly. Boundary layer shear forces and the reactant pressure aid in transporting the water through the channels and the tunnel region until the water exits the fuel cell through the outlet header.
Numerous techniques have been employed to remove water from the tunnel regions and headers of the fuel cell. These techniques include pressurized purging, gravity flow, and evaporation. A pressurized gas purge at shutdown may be used to effectively remove water from the tunnel regions and headers of fuel cells. However, the purge increases shutdown time of the stack and does not remove any water formed from condensation after the purge. Positioning of the stack appropriately may allow gravitational forces to remove water from the tunnel regions and headers. However, gravitational removal of water may be limited to substantially flat surfaces, surfaces having at least a minimum diameter, and surfaces of low energy. Capillary forces of the tunnel region and self wetting of the plurality of seams in the bipolar plates also militate against gravitational removal of water. Water removal by evaporation has been an undesirable technique as well. Evaporation may require costly heaters to be placed in the headers and may lead to an undesirable drying of the fuel cell stack. Additionally, evaporation may only be performed during operation of the fuel cell stack. A dry fuel cell stack militates against proton conduction and prompt starting.
Water that accumulates in the tunnel regions of the fuel cell in sub-freezing temperatures may freeze after the fuel cell is shut down. Frozen water in the tunnel regions and the headers of a fuel cell may prevent the fuel cell from restarting or result in poor performance of the fuel cell until a desired operating temperature is reached.
In addition to water produced from the fuel cell itself, water may enter the tunnel region of the fuel cell from an inlet or the outlet header. During fuel cell operation, liquid water may collect on the edges of the fuel cell plates that form the inlet and outlet headers. Also, the humid environment necessary for the operation of the fuel cell promotes water condensation in the headers after fuel cell shutdown. As the water accumulates on the edges of the fuel cell plates, the water also wicks along the edges forming the headers. The condensed water may wick into the tunnel region, causing one of a self wetting the tunnel region of the bipolar plates of the fuel cell and the formation of a plurality of menisci along an edge of the tunnel region.
There is a continuing need for a cost effective bipolar plate for a fuel cell that facilitates the removal of water from the header of the fuel cell stack, militates against water entering the tunnel regions of the bipolar plate, and militates against the tunnel regions of the fuel cell stack from becoming blocked with frozen water.